This invention relates to martensitic memory alloys and, more particularly, to conditioning an annealed martensitic nickel/titanium alloy to improve its service life and elongation activity under high tensile stress operating conditions.
Alloys of nickel and titanium in which the two elements are present in roughly the same molar proportions have been demonstrated to have martensitic memory properties rendering them highly useful in control devices and other services in which temperature actuation is desirable. When placed under stress, an alloy roughly corresponding to the formula NiTi undergoes a martensitic phase transformation in a relatively narrow temperature range with a resultant change in dimension. This dimensional change is negative with respect to temperature. Thus, if an NiTi wire is under tension and is cooled from a temperature above the martensitic transformation range, it will elongate when a critical temperature range is reached. Conversely, when the wire is heated from a temperature below the martensitic range, it will shorten in a temperature range in which the phase transformation is reversed.
In such thermal cycling of the wire there is a hysteresis effect in that the major share of the reverse transformation takes place in a temperature range somewhat higher than the temperatures at which the major share of elongation takes place. This phenomenon is illustrated in FIG. 1. Thus, on cooling, conversion of austenite to martensite commences at a temperature designated M.sub.s and conversion to martensite is essentially complete at a temperature designated M.sub.f. On heating, conversion of martensite beings at a temperature A.sub.s (A.sub.s &gt;M.sub.f) and conversion to austenite is complete at a temperature designated A.sub.f (A.sub.f &gt;M.sub.s). The phase transformation associated with elongation is accompanied by the release of heat energy and the reverse transformation is accompanied by an absorption of heat.
Because of their unique property of elongating and reversibly foreshortening over a relatively narrow temperature range, martensitic memory alloys, such as nickel/titanium, have found application as thermostatic elements in control devices and as means for the conversion of heat energy to mechanical energy in devices for performing work. Where the alloy is in the form of a thin wire, for example, it may be very rapidly heated or cooled to cause sharp changes in dimension. The practical utility of such a device is enhanced by the extent of this change in dimension. The martensitic elongation activity of these alloys, defined as the ratio of change in length to length (.DELTA.L/L) expressed as a percentage, may range range as high as 2-6%.
A feature of nickel/titanium martensitic alloys which may tend to limit their practical utility is the propensity for their martensitic transformation temperature ranges to be near room temperature. As a consequence, the alloy may undergo phase transformations and resultant elongations and foreshortenings due to ambient variations alone. The effective transformation temperature range of such alloys can be altered, however, by placing the alloy under stress. Thus, for example, if a nickel/titanium alloy wire is placed under a relatively high tension, the temperature ranges over which the phase transformation takes place may be increased by 70.degree. C. or more. The general character of the elongation versus temperature curve remains similar to that deposited in FIG. 1 but the ranges over which austenite/martensite transformations occur are displaced to the right if plotted as in FIG. 1. When cycling under stress, the temperature at which conversion of austenite to martensite begins is designated as M.sub.d rather than M.sub.s, and the temperature at which conversion to austenite begins is designated as A.sub.d rather than A.sub.s.
Although stress is known to be effective in raising the temperature ranges over which martensite/austenite transformations occur, the feasibility of realizing substantial increases in the operating temperature of a nickel/titanium device may be limited by the tensile strength of the alloy, by the service life of the alloy at high tensile stress, and the effect of high tensile stress in reducing the elongation activity of the alloy. Additionally, the application of high tensile stress may cause the alloy to creep at elevated temperatures or undergo progressive elongation with repeated cycling under service operating conditions.
A number of processes have been proposed for conditioning nickel/titanium martensitic alloys with the purpose of improving their operating characteristics. Thus, for example, Willson et al. U.S. Pat. No. 3,652,969 describes a process in which the stability of a nickel/titanium control element is improved by repeatedly cycling it through its martensitic transformation range at a load greater than the load to be utilized in service. Thus, Willison et al. describe cycling the element under a stress of 40,000 psi where the service load is 20,000 psi. This process, however, relates to relatively low strength alloys and is, therefore, not directed to the problem of increasing service life and maintaining elongation activity under very high tensile stresses in the range of 175,000 psi or greater.
Wang, Journal of Applied Physics, Vol. 44, No. 7, July 1973, p. 3013, describes a method by which the repeatability of a martensitic alloy is improved by cycling it partially through its transformation range, while maintaining it under a tensile stress just sufficient to deform the material to the limit of its easy plastic flow region. For a typical nickel/titanium alloy comprising on the order of 54.3% by weight nickel, annealed in accordance with the method described in my copending application Ser. No. 427,164, such stress would be on the order of 85,000 psi. However, Wang's object is merely to enhance the reversibility of the alloy transformations and the Wang method is not directed to improved service life or maintenance of high elongation activity under extra high tensile stresses.
In my aforesaid copending application, I have described a process for increasing the tensile strength of a martensitic alloy of titanium and nickel by maintaining the alloy under a tensile stress of between about 30,000 and about 100,000 psi, while annealing it at a temperature above a first diffusional phase transformation temperature. This process is effective not only to increase the tensile strength of the alloy but to stabilize it against progressive elongation even under severe operating conditions, and to maintain its elongation activity at a level of at least about 2% at high tensile stress. The product of the annealing process of the aforesaid application is highly satisfactory for many practical uses. A need has remained, however, for further improvement in the service life of the alloy under high tensile stress conditions, and for further improvement in elongation activity.